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Transcript
Free Radical Substitution Homolytic Fission Substitution Rxn (free radical substitution) • Is a chemical reaction in which an atom or group of atoms in a molecule is replaced by another atom or group of atoms Mechanism of reaction • Is the detailed step by step description of how the overall reaction occurs Methane Chloromethane H + Cl Cl Chlorine = Cl + H Cl Hydrogen Chloride Simple mechanism Chloromethane Methane H Hydrogen and Chlorine have swapped places Substitution Hydrogen Chlorine Chloride Cl Cl Stage 1 Initiation Getting Started Ultra violet light breaks the bond Chlorine molecule Cl2 2 Chlorine radicals each with an unpaired electron Both species are the same Called Homolytic Fission Stage 2 Propagation Keeping it going Methane H Methyl radical Chlorine radical Cl The chlorine radical pulls the hydrogen and one electron across to it. Hydrogen chloride Lets put in the 2 electrons in this bond The methyl radical is now free to react with a chlorine molecule Chlorine radical Cl Cl Methyl Chlorine Chloromethane radical Chlorine radical can now go and react with a methane molecule Stage 3 Termination Grinding to a halt • Three different ways this can happen Cl Cl Chlorine molecule Chlorine Chlorine radical radical Reaction stops No free radicals to keep it going Cl Chlorine Methyl Chloromethane forms radical radical Reaction stops Because there are no free radicals to keep it going Ethane Methyl stops Methyl Reaction because no free radicals radical radical produced to keep it going The formation of ethane proves that this is the mechanism Reaction speeded up by sources of free radicals such as tetramethyl lead. Proof of mechanism •Small amount of ethane detected •Not initiated (start) in dark needs UV light •Tetra methyl lead decomposes to form methyl free radicals, if Tetra methyl lead added it increases rate of reaction Pb (CH ) = Pb + 4 CH º THERFORE Methyl radicals are used in reaction •Halogenated alkanes are used as flame retardants 3 4 3 Step1: Initiation UV light stimulates rxn. Cl-Cl molecule splits equally (homolytic fission) Step 2. Free Cl° atoms (RADICAL) Propagation attacks methane and forms HCL Step 3. Methyl free radical Propagation attacks a Cl-Cl molecule and forms chloromethane. Chain rxn continues. Step 4: Cl°+ Cl° =Cl-Cl Termination Cl°+ °CH3= CH3Cl CH3+ CH3= C2H6 Tetramethyl lead is added to speed up the rxn • It supplies the solution with methl free radicals. • Evidence for free radicals comes from small amounts of ethane being found in the solution • Halogenation of alknaes makes them more flame resistant Addition Reaction (pg. 367) • When two substances react together to form a single substance Addition Reaction Mechanism and evidence for Heterolytic Fission Step 1 Polarising of the bond in bromine Ethene Concentration of negative charge Because 4 electrons in this area Bromine Br2 δ+ δ- Moves in this direction At this point the negative charge of the double bond in ethene forces the electrons to the right Br and it becomes δ− and other one becomes δ+ Step 2 Heterolytic Fission Occurs The two electrons of the bond have been forced across to the right Br making it Br- while the other is Br+ Br+ Br - δ+ δ- At this point the negative of thebeen double Thecharge Br2 has split into bond in ethene forces the + 2 different species i.e. Br electrons to the right Br andand Brit- becomes this is called δ− and other one becomes Heterolytic Fission δ+ Step 3 Formation of the Carbonium Ion At a lotofof Thethis twopoint electrons thethings bond have beenatforced acrosstime to the happen the same Br Br+ + Carbonium ion Cyclic bromium ion right Br making it Br- while the •other Theis two Br+ electrons are pulled to the Br+ us put in the two Br • A bondLet is formed electrons of this bond • one of the bonds between the two carbons disappears • The lower carbon becomes The split +veBr because it has lost aninto 2 has been electron species i.e. Br+ 2 different - this • TheBr two hydrogens on the and is called upper carbon move to make Heterolytic Fission way for the Br Step 4 Attack on carbonium ion by Br The negative bromide ion is attracted by the positive carbonium ion Br + Br Br - Carbonium ion 1,2 dibromoethane Called Ionic Addition because the species are ions when they add on The two electrons of the bromide ion are used to form the bond The two hydrogen atoms move round to allow the Br in The negative and positive cancel each other out Step 5 Proof of mechanism Br + Br - Cl Cl Br Proof of the mechanism is that if there are Cl- in the environment then some 1-bromo, 2-chloroethane will be formed. This can be identified by its different Relative Molecular Mass Step1: σ+Br-σ-Br- Step 2. Ionic addition Step 3. Ionic addition Step 4: Termination Carbon double bond is region of high edensity. Br2 becomes polar as comes close Br2 splits into ions.heterolytic fission because Br+ and Br- created Br+ molecule attacks double bond and forms cyclic bromonium ion/ carbonium ion Br- now attacks the carbonium ion Hydrogenation • Adding of hydrogen's into a molecule (addition) • Occurs in manufacture of margarine • Add hydrogen into double bonds causes oils to become solid • Unsaturated fats are better for you that saturated fats Evidence for the carbonium ion • When bromine and chlorine ions present • Ethene forms 1-bromo-2-chloroethane as well as 1, 2 dibromoethane Polymerisation rxns • Molecules that contain double bonds undergo addition to become less unsaturated (addition polymers such as polythene and polypropene) Polymerisation Reactions • Example of an addition reaction • Ethene molecules add together • Polymers are long chain molecules made by joining together many small molecules + = Polymers • Commonly reffered to as plastics • Polyethene used for plastic bags, bowls, lunch boxes, bottles etc • Polypropene is used in toys, jugs, chairs etc • Crude oil is raw material for their manufacture Elimination reactions Elimination reactions • When a small molecule is removed from a larger molecule to leave a double bond in the larger molecule Elimination rxns • a compound breaks down into 2 or more simpler substances • Double bond created • only one reactant AB A + B Elimination reactions • Ethene is made from ethanol from removing water using AlO as catalyst • Elimination reaction is one in which a small molecule is removed from a larger molecule to leave a double bond in the larger molecules • Dehydration reaction • Only need to know dehydration of alcohol Elimination reaction • Dehydration of an alcohol is an example of an elimination reaction • In this reaction, a larger alcohol molecule reacts to form a smaller alkene molecule and an even smaller water molecule • The change in structure is from tetrahedral to planar Dehydration of ethanol • Ethanol is dehydrated to ethene • This reaction is used in the preparation of ethene Dehydration of ethanol to ethene Reaction conditions • Heat • Aluminium oxide catalyst Preparation of ethene Elimination rxn • Is when a small molecule is removed from a larger molecule to leave a double bond in the larger molecule • Alcohol =water + alkene • Dehydration reaction since water is removed • Ethanol=ethene + water • 2 methanol +sulphuric acid =methoxymethane ether +water C. Decomposition 2 H2O(l) 2 H2(g) + O2(g) Redox reactions Redox reactions • These reactions involves oxidation and reduction reactions • The removal or addition of lectrons from the molecule -3 -2 -1 0 Reduction Receives electrons Reducing agents give electrons 1 2 3 Oxidation Looses electrons Oxidation agents take electrons Redox reactions of primary alcohols • Primary alcohols react with oxidising agents such as potassium manganate(VII) or sodium dichromate(VI), forming the corresponding aldehyde • For example, ethanol reacts forming ethanal • Ethanal is also formed in the metabolism of ethanol in the human body Redox reaction • Primary alcohol oxidised to an aldehyde • Oxidising agent: sodium dichromate or potassium permanganate • The oxidising agent must be limited to prevent the aldehyde from being further oxidised to an carboxylic acid Reaction of ethanol with sodium dichromate(VI) Reaction of ethanol with sodium dichromate(VI) • This reaction is used in the preparation of ethanal • Reaction conditions: heat, excess ethanol, acidified sodium dichromate(VI) solution • The aldehyde is distilled off as it is formed in order to prevent further oxidation to ethanoic acid Preparation of ethanal Oxidation of primary alcohols • Primary alcohols such as ethanol are oxidised to the corresponding aldehydes, which can be further oxidised to the corresponding carboxylic acids. Oxidation of ethanol H H H C C H OH H + H Na2Cr2O7 / H H C H O C H Na2Cr2O7 / H+ H H C H O C OH Reaction of ethanol with sodium dichromate(VI) • This reaction is used in the preparation of ethanoic acid • Reaction conditions: heat, excess acidified sodium dichromate(VI) solution • The reaction mixture is refluxed in order to bring about oxidation to ethanoic acid Preparation of ethanoic acid Reflux followed by Distillation Oxidation of secondary alcohols • Secondary alcohols such as propan-2-ol are oxidised to the corresponding ketones, such as propanone • Unlike aldehydes, ketones are not easily oxidised, and so no further oxidation takes place Oxidation of propan-2-ol OH O + H3C C CH3 H Na2Cr2O7 / H H3C C CH3 Combustion of organic compounds • Most organic compounds burn in air, forming carbon dioxide and water • The structure of the compounds’ molecules is completely destroyed, with the carbon and hydrogen atoms in each molecule being oxidised • Combustion is exothermic, and ethanol is used as a fuel where it can be produced cheaply Non-flammable organic compounds • Fully halogenated alkanes such as bromochlorodifluoromethane are nonflammable • Because of this they can be used in fire extinguishers and as flame retardants • For environmental reasons, the use of many of these substances is being phased out Reduction of aldehydes and ketones • Aldehydes and ketones can be reduced to the corresponding alcohols, using hydrogen passed over the heated surface of a nickel catalyst • For example, ethanal is reduced to ethanol Reduction of ethanal to ethanol Reduction of propanone to propan-2-ol ENERGY PROFILE one step reaction ONE STEP transition state TS energy maximum E N E R G Y activation energy Ea obtained from heat (collisions) heat of reaction DH exothermic starting material (releases heat) product REACTION COORDINATE ( follows the progress of the reaction ) opposite is endothermic Reactions as acids Reactions of alcohols with sodium • Alcohols react with the reactive metal sodium, forming a sodium salt and hydrogen • For example, ethanol reacts with sodium forming sodium ethoxide and hydrogen Reaction of ethanol with sodium Acidic nature of the carboxylic acid group • Ethanoic acid is a far stronger acid than ethanol • This is because its anion is much more stable than that of ethanol • This enables it to lose a hydrogen ion more readily • The stability of the ethanoate ion is due to electron delocalisation (as in benzene) Reactions of carboxylic acids as acids • Carboxylic acids react with: • Magnesium, forming a magnesium salt and hydrogen • Sodium hydroxide, forming a sodium salt and hydrogen • Sodium carbonate, forming a sodium salt , carbon dioxide and water Reaction of ethanoic acid with magnesium • Acid + metal → salt + hydrogen 2 CH3COOH + Mg → (CH3COO)2Mg + H2 ethanoic acid ethanoate magnesium Reaction of ethanoic acid with sodium hydroxide • Acid + Base Water → Salt + CH3COOH + NaOH → CH3COONa + H2O ethanoic acid sodium ethanoate Reaction of ethanoic acid with sodium carbonate Acid + Carbonate → Salt + Water + Carbon dioxide 2CH3COOH + Na2CO3 → 2CH3COONa + H2O + CO2 ethanoic acid sodium ethanoate